How to convert a string to a symbol ?

Hello guys ,

I just wanted to convert this string :

'x & 1'

to a symbol ,i tried to use sym() function ,which seemed not to work ,obviously ,it doesn't seem to symbolize some characters ,like ampersands ,asterisks ,arithmetic operators , etc...

So any help how can i convert the above string to a symbol ?!

Much Regards

Answer 1

[Ahmed A. Selman]

@Jacky (the actual expression is : (x>=1 & x<9)). This is the question!

This is a logical comparison statement. The meaning of (&) - Logical AND in Matlab in comparisons is

&&

So try this

   if (x>=1 && x<9) 
     y=2*x+12;% here is what to do when the condition is fulfilled 
   else 
     y=0;% here is what to do when the condition is not fulfilled
  end

There is a (symbolic) programming tools in Matlab, initialized with

   sym 
or 
   syms

e.g.,

   syms x y z

   whos
    Name      Size            Bytes  Class    Attributes

    x         1x1                58  sym                
    y         1x1                58  sym                
    z         1x1                58  sym

the same output we''ll get when using

   syms ('x', 'y','z')

and x, y, z (or any other argument) must be variables name. Variables names must not contain special characters such as '&'.

After we defined the syms above, we can try this:

   f=(cos(x)+sin(y))/z 
   fPrime=diff(f,x)
   fPrime =

   -sin(x)/z

where fPrime is the differentiation of f with respect to the variable x... and so on.

Converting (string to symbol in Matlab) is still not clear to me.

Regards :)

[Walter Roberson]

Both & and && are logical comparisons in MATLAB. && is "shortcut and" which only works on scalars and does not evaluate the right-hand side if it already knows the answer from the left-hand side.

The difficulty being had is what I described above: the MuPAD, the symbolic language, does not use & or && at all: it uses "and"

Answer 2

[Ahmed A. Selman]

sym and syms are used to declare symbolic variables, see:

http://www.mathworks.com/help/symbolic/symbolic-variables-expressions-and-functions.html?searchHighlight=symbolic

Please specify your idea about (converting a string to symbol in Matlab), do you mean (string to a variable), as in

x=1
y=-90

if so, Matlab variables must not contain special characters as (~!@#$%^&*)..etc. Or you mean text properties in figures? Then Refer to http://www.mathworks.com/help/matlab/ref/text_props.html.

If it was translating a string into an expression, see the functions

regexptranslate
regexp

[Jacky]

Sorry for not being specific in my question .

I mean if i have a string which contains one or more special character ,and i need to convert this string into a symbol ,what should i use in such case ?!

[Walter Roberson]

What result are you aiming for, and what are you getting instead?

[Jacky]

What i want to do is ,simply i want to take from user a string containing a function to plot and store it in a variable ,and in another variable storing the limits of the function ( for example 1<x<=5 ) ,of course this won't be done on a single function ,instead it will be like constructing a piecewise function that plots multiple functions each with its limits . So i need to take the limits from user and symbolize it .

[Walter Roberson]

When you use

sym('x & 1')

what result are you seeing?

Question: would x&1 be the formula or the limit? Were you meaning

 sym('x < 1')

?

[Jacky]

sym('x&1') it doesn't accept the character '&' .

x & 1 is something like the limits i want to do ,the actual expression is : (x>=1 & x<9)

Answer 3

[Walter Roberson]

MuPAD does not use & for logical conjugation. See http://www.mathworks.com/help/symbolic/logical-operations-1.html

Answer 4

[Jacky]

Perfect ,found what i was looking for ,thx alot guys for the help :)